An ink-jet recording apparatus has been known, which has a recording head for discharging ink as droplets onto a recording paper to print an image. The ink-jet recording apparatus is provided with at least an ink container containing ink, to supply the ink from the ink container to the recording head. In an example, the recording head is provided with at least a nozzle and an oscillation plate driven by a piezoelectric element. Making use of pressure change in the nozzle, which is caused by oscillating the oscillation plate, the recording head sucks the ink from the ink container into the nozzle, and discharges the ink through an ink outlet of the nozzle.
Because the ink is a consumable material, the ink container is often formed as a cartridge that is removably attached to the ink-jet recording apparatus, so as to make it easy to supplement the ink-jet recording apparatus with the ink. Such a cartridge type ink container, hereinafter called the ink cartridge, is replaced with another that is fully filled with the ink, when the ink contained in the ink cartridge is used up. In an ink cartridge loading section of the recording apparatus, an ink supply needle is disposed for supplying the ink from the ink cartridge to the recording head. The ink cartridge and the nozzle of the recording head is connected through an ink supply path, including the ink supply needle, to let the ink flow through the ink supply path.
An ink-jet recording apparatus disclosed in Japanese laid-open Patent Application No. 2003-300331 uses an ink cartridge that consists of a flexible ink bag and a case protecting the ink bag. If the ink is exposed to the air, the air will be solved in the ink, forming air bubbles in the ink, or some components of the ink react with oxygen, deteriorating the ink. To keep the air out of the ink, the ink cartridge uses the air-tight ink bag.
It is known in the art that the pressure inside the nozzle of the recording head, hereinafter called the nozzle internal pressure, is kept negative relative to the atmosphere, in order to prevent the ink leakage through the nozzle, which would otherwise be caused by the weight of the ink. Where the ink cartridge is placed above the recording head, the nozzle internal pressure is raised by the weight of the ink contained in the ink cartridge, so it cannot keep the negative value relative the atmospheric pressure without any countermeasure. According to the above prior art, the air in a room between the ink bag and the case is sucked by a suction pump to reduce the pressure in the room, so that the nozzle internal pressure is kept negative relative to the atmospheric pressure. Hereinafter, the pressure in the room between the ink bag and the case will be called the case internal pressure.
As the ink in the ink bag is consumed, the pressure applied to the nozzle by the ink weight decreases, so the negative pressure in the nozzle would become too large if the case internal pressure is kept at the initial negative value. In that case, the ink discharged from the nozzle would be improperly reduced, lowering the print density improperly. To avoid this problem, the above-mentioned prior art suggests providing a pressure sensor for measuring the nozzle internal pressure, to control sucking amount of the suction pump depending upon the measured value, so as to keep the nozzle internal pressure in a proper range.
However, the method of controlling the nozzle internal pressure while measuring it by the pressure sensor has a disadvantage that there is a delay time from the measurement by the pressure sensor to the pressure control based on the measured value. The delay of the pressure control can cause fluctuation in the ink discharging amount. Instable discharging of the ink results in uneven density of the printed image and lowers the image quality. This problem is conspicuous especially in an image of a high printing duty ratio, like a solid image.